Just got a 1999 1.6L Sentra with 99000 miles. The car runs great.
The engine is smooth and quiet, it idles beautifully and all four plugs
look fine. Gas mileage is fine too. However, the check engine light
comes on setting a P0420 code (catalyst effeciency below threshold,
bank 1). If I turn the light off, it comes back on in 2 or 3 drive
cycles, so the problem is persistent. I know that P0420 is set when
the ECM sees the rear O2 sensor tracking too closely with the front O2
sensor. So after driving for an hour and getting everything to normal
operating temperature I put it on a scanner and ran the engine at about
2100 rpm for a minute or two to watch the O2 sensors. I did the test
four times. Between tests I let the engine idle for a minute.
All 4 times the front O2 sensor switched about every second or two
from about 200mV LEAN to about 800mV RICH. That seems normal. The
rear O2 sensor behavior was puzzling though. The first two times the
rear O2 sensor never switched but rather stayed very constant at about
400mV LEAN and very slowly rose to about 500mV LEAN. The O2 heater was
always OFF. The next two times the rear O2 sensor again never switched
but was now at 700mV RICH and rose very slowly to about 800mV RICH.
The O2 heater was now ON. I'm not sure how to interpret these results.
Is there a problem with the rear O2 sensor? Also, why is the light
coming on if the two O2 sensors are operating so differently? I'd
appreciate any thoughts as to what's going on and what the problem is.
Many thanks.

Look at http://www.autoshop101.com/ for the Technical Article 37, it may
help.

The ECU is probably expecting a greater difference. I'd


No answers, but some thoughts:
Observing your data, the issue comes down to 3 possibilities:
The front O2 is bad
The rear O2 is bad
The catalyst is bad

The front O2 may be bad, but the ECU is not throwing a code . . . . and
this is a normal occurence. I have the same car, 1999 Sentra GA16DE
motor, and I recently replaced the front O2 even though I had no check
engine light. Look at my thread here;

http://groups.google.com/group/alt.a...72904fb1668cb9

My front O2 was bad and the ECU did not know. Also, my gas mileage was
still good. I went through replacing plugs and wires before I came to
the O2.
My Sentra is slightly different from yours in that mine does not have a
rear O2 sensor. Yours is a california spec emissions vehicle and mine is
a federal spec. Fed spec vehicles usually dont have a rear O2 sensor.

That cat may also be bad. Since it is the most expensive item to
replace, I would start with the O2s first.
Usually, the front O2 goes bad first. It's in a more hostile environment
sitting right up there in the exhaust manifold.

You can diagnose further by looking for symptoms. I was able to find out
that my front O2 went bad because the car lost power. Since the car is
new to you, you may not notice what's missing. Does your car feel peppy
from idle to about 2500-2700 rpms, then sort of lose its ability to
accelerate? If so, then I'd go with the front O2 sensor, because it
tells the ECU when\how to fire the injectors, hence play with the car's
power curve. The rear O2s reason for being is just to make sure that the
cat is working.

Hope this helps.

CD

That's a nice site, but it's not perfect. For example, it says that all
OBDII cars have a front and rear O2 sensor. My car is OBDII and it only
has a front O2 Sensor.

CD

Thanks for the helpful ideas. Yes it's a California set-up (two O2
sensors and two cats). It is new to me so I can only compare it to my
1987 Sentra and it sure runs a lot stronger. But I'll goose it little
more at speed as you suggest and see how it feels. Otherwise, it runs
quietly and smoothly, idles beautifully and all four plugs look fine.
So I'd never have guessed a problem.
I put it on the scanner again and yet again at 2100 rpm the front O2
sensor switches often while the rear O2 sensor doesn't switch. That's
as it should be. But at idle both O2 sensors switch so it appears that
the code is being set at idle. Maybe the front O2 sensor isn't
switching fast enough at idle or the cat is losing efficiency and can't
handle idle emissions. The front O2 sensor hypothesis seems more
plausible, and cheaper too. That Autoshop site has a lot of good info
especially on theory of operation. Thanks again guys.

Have you tried connecting and disconnecting the O2 sensors, just to make
sure the connections are alright? Or try driving it with the revs up to
get the exhaust real hot, as to burn any deposits that may be
accumulating on the sensors or even on the cat converter.

By the way, in the autoshop place, when you take the online tests, Scan
tool diagnosis, there is a bunch of problem scenarios with scan readings
and the right answers. It is worth your while.

Good luck.